Geochemistry and Pb isotopic proof for sources of heavy metal(loid)s in Late Cretaceous sandstones from Eastern Pontides (NE Turkey)

Upper Cretaceous sedimentary rocks are widely exposed around Trabzon, Giresun and Gümüşhane in the eastern Pontides. The sedimentary rocks are mainly composed of sandstones and marls, and the thicknesses of these rocks range from 170 m to 400 m. This paper focused on Late Cretaceous sandstones of Trabzon, Giresun and Gümüşhane regions to identify the concentrations, sources, deposition environment and conditions of certain heavy metal(loid)s (Zn, Pb, Cu, Ni, As, Co, Mo and V). Enrichment factor (EF), geo-accumulation index ( Igeo ), Pollution index (Pi), Pollution load index (Pl n ), Pb isotope and multivariate statistical analysis were performed to elucidate the pollution levels and sources. The mean concentrations of Zn, Pb, Cu, Ni, As, Co, Mo and V were 16.5, 10.8, 9.4, 13.2, 2.1, 4.4, 0.2 and 21 mg/kg, respectively in the Trabzon area, 35, 5.8,18.8, 102, 3.2, 18.6, 0.5 and 109 mg/kg, respectively in the Giresun area, and 36, 8.9, 14.7, 82, 3.9, 16.9, 0.3 and 106 mg/kg, respectively in the Gümüşhane area. In general, the evaluation indices exhibited that the sandstones were moderately polluted by As in the Trabzon area, and strongly polluted by Ni and As, moderately polluted by Co and V in the Giresun and Gümüşhane areas. The results of multivariate statistical analyses indicated that As and partly Pb might be originated from anthropogenic sources, whereas other metals were derived from geogenic origin. Lead isotopic analysis demonstrated two characterized signatures of the pollution sources in the sandstones, one related to geogenic origin and the other to coal, gasoline and pesticides. The sandstones were deposited in transition – marine environment under oxic – weak oxic conditions and paleoclimatic conditions ranged from arid to moist during Late Cretaceous period. study to: (1) Define the concentrations and vertical change of 8 heavy metal(loid)s (Zn, Pb, Cu, Ni, As, Co, Mo and V) in the Upper Cretaceous sandstones


Grain size analysis
For the well-cemented sandstones, thin sections have to be used and several hundred grain sizes measured with an eyepiece graticule and point counter, as detailed is in McManus (1988).

Physicochemical analysis
The total organic carbon (TOC) content of the sandstone samples was defined by oxidation with acidified K2Cr2O7 and back titration with ferrous ammonium sulphate. They were soaked in water with an aliquot of 10% sodium hexamtaphosphate solution and lightly agitated for about 1 h to increase disaggregation. The samples were then washed over an 80 μm sieve and oven-dired at a temperature 0f 60 o C. The percent mud of the samples was calculated by multiplying the mass of the 80 μm fraction and the initial sample mass minus water content. The TOC content of sandstones was determined by Bernard calcimeter method (Guitián and Carballas, 1976).

Geochemical analysis
Major and trace elements were determined on the 33 sandstone samples (6 samples for Trabzon, 10 samples for Giresun and 17 samples for Gümüşhane), at the commercial Acme Analytical Laboratories Ltd (now Bureau Veritas Minerals) in Vancouver, Canada. Major and element (Al2O3, Zn, Pb, Cu, Ni, As, Co, Mo and V) compositions were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) using 0.2 g of sediment fused with 1.5 g of LiBO2 and dissolved in 100 ml of 5% HNO3. Loss on ignition (LOI) was determined using dried samples heated to a temperature of 1.000 °C for 15 min. The detection limits were in the range of 0.001 wt% to 0.1 wt% for major element oxides and 0.1 ppm to 10 ppm for trace elements and 0.01 ppm for Hg. Calibration and verification standards together with reagent blanks were added to the sample batches. STD SO-18 was certified in-house against 38 certified reference materials, including CANMET SY-4 and USGS AGV-1, G-2, GSP-2, and W-2 as known external standards. The analytical accuracy is better than 4%.

Pb isotope analysis
Isotopic analysis of Pb was performed at the commercial Acme Analytical Laboratories Ltd. in Vancouver, Canada. The prepared sample is digested with a modified aqua regia solution of equal parts concentrated HCl, HNO3, and DI H2O for 1 h in a heating block or a hot water bath. The sample is made up to volume with dilute HCl. Sample splits of 0.5, 15, or 30 g were analyzed. Pb isotope add-ons (+ISO) Pb204, Pb206, Pb207, and Pb208 are suitable for geochemical exploration of U and other commodities where gross differences in natural to radiogenic Pb ratios are beneficial. Isotope values can be obtained from the concentrations and intensities. Sample splits of 0.5, 15, or 30 g were analyzed. Pb isotopic analyses were performed on three samples (D1, D17, D21) from   Trabzon area, five samples (C11, C18, A14, A28, A36) from Giresun area and six samples (N17, N37, N40, R34, M30) from Gümüşhane area.

Enrichment factor (EF)
EF is commonly used to assist the determination of the level heavy metal(loid) pollution. Furthermore, Thang et al. (2018) suggested that this index is useful in evaluation the anthropogenically introduced heavy metal(loid) and the equation is: Where, Ci is the ingredient of the studied element in the sandstone; Cref is the concentration of a reference element (Sc) for normalization objectives, Bn is the background concentration of the heavy metal(loid) in the upper continental crust (UCC), and Bref is the background concentration of the reference element (Sc) in the UCC (Taylor and McLennan, 1985).

Geo-accumulation index (Igeo)
Igeo, used initially by Müller (1969;1981), has been generally used to define the pollution level of heavy metal(loid) in sediments and the equation is: Where Cn indicates the concentrations of heavy metal(loid) n in the analyzed sample, Bn is the reference concentration of the element in the background sample (Wedepohl, 1995) and the factor (1.5) is used to minimize the influence likely changes in the background values, as a result of lithogenic input.

Pollution index (Pi) and Pollution load index (Pln)
Pi is the ratio of single heavy metal(loid) contents in sediments and the background value. Pi is calculated with the following equation (Cheng et al., 2007): Where Cn is the concentration of the studied heavy metal(loid), Cref is the reference value.
Pln is used for definition of total heavy metal(loid) pollution in sediments certain site and mean of Pi of all the distinct heavy metal(loid)s present in the study areas (Tomlinson et al., 1980;Rana et al., 2016;Raj et al., 2019).
Pln is calculated with the following formula (Cheng et al., 2007): Where Pi is the monomial pollution index of heavy metal(loid), (Pi)max is the maximum value of the monomial pollution indices of all heavy metal(loid)s.

Multivariate statistical analysis
Pearson's correlation matrix (PCM) and principial component (PCA) was used to decode the interrelations among heavy metal(loid)s and physicochemical parameters. These analyses were applied with a statistical software SPSS 22.0. PMC analysis commonly determined the relation among the heavy metal(loid)s and verified the conclusions of multivariate analysis (Tahri et al., 2005;Liao et al., 2017). In addition, this analysis as used to understand pollution sources (anthropogenic and geogenic).
The heavy metal(loid) concentrations in sediments from Trabzon, Giresun and Gümüşhane areas were summarized in composition of the studied sandstones was not importantly influenced by the grain size of the sandstones (except Zn, Ni, Co, Mo and V in the Trabzon area).

Pollution assessment of heavy metal(loid)s
Enrichment factor (EF)  (Taylor and McLennan 1985). The means of EF were in the order of Mo < Zn <V < Cu < Co < Pb = Ni < As in the Trabzon area, Pb < Mo < Zn < Cu < Co = V < As < Ni in the Giresun area and Mo < Zn < Cu = Pb < Co = V < As < Ni in the Gümüşhane area. A threshold of 2 was admitted to descriptive whether a definite metal was enriched in soil. If a local reference element is used to compute EF, this threshold could be decreased to 1.5 (Hernandez et al., 2003;Roussiez et al., 2005;N'guessan et al., 2009;Benabdelkader et al., 2018;Wu and Probst, 2021). The mean EF of the Ni for Giresun area and As, Ni for Gümüşhane area were higher than the threshold of 2. In the Trabzon area, As for D12 and D21 samples was higher than threshold (Fig. 5a). In the Giresun area, As for C18 and A14 samples, Ni for all samples were higher than threshold ( Fig. 5b). In the Gümüşhane area, Co for N3 sample, Pb for R34, As for N17, N33, N37, N40, R15, R31, R34, M30 and M33, and Ni for about all samples (except N50, R34, M30 and M33) were higher than threshold ( Fig. 5c). The EF values of Mo had been calculated as very low in all the studied samples.  Pb, Cu and Co were negative in the Gümüşhane area, indicating that the sandstones were polluted by Ni, As and V (unpolluted to moderately polluted by As, V and moderately polluted by Ni), and unpolluted by Zn, Cu, Co and Pb (Fig. 6c). In additionally, Fig. 5d showed that the sandstones in the Giresun and Gümüşhane areas were polluted by Ni in approximately same amount.

Pollution index (Pi) and Pollution load index (Pln)
The Pi rates of the heavy metal(loid) of the sandstones in the Trabzon area exhibited unpolluted to low polluted and varied as: Zn (0. indicating that the sandstones were unpolluted by these metals, and Cu and As were between 1 and 2 (1.0 and 1.53, respectively), indicating that the sandstones were low polluted by Cu and As (  Fig. 7d). In the Gümüşhane area, Pln values of Zn, Pb and Cu were between 1 and 2 (1.15, 1.29 and 1.25, respectively), indicating that the sandstones were low polluted by these metals, Co and V were between 2 and 3 (2.05 and 2.51, respectively), indicating that the sandstones were moderately polluted by Co, V, and As and Ni were above 3 (4.07 and 6.15, respectively), indicating that the sandstones were strongly polluted by As and Ni (Table 4 and Fig. 7d).

Lead isotopic tracing in the sandstone
Lead isotopes provide an effective tool for determining the pathways and sources of Pb pollution because the isotopic composition of Pb is not importantly impressed by physico-chemical fractionation processes Veysseyre et al., 2001;Komárek et al., 2008). Therefore, Pb isotope analysis is often used to define whether content of metals in sediments of anthropogenic or geogenic. Lead in sediments is possibly a mixture of natural lead from the weathering of host rocks and anthropogenic Pb. Lead is present in widespread various rock kinds range from about 30 mg/kg to 1 mg/kg among igneous rocks, black shale, evaporitic rocks, rhyolite and granitic rocks, respectively (Wedepohl 1995;Cheeme et al., 2020). These rocks included radioactive elements have high concentrations of Pb than sedimentary rock. In same time, sedimentary rocks also include Pb contents ranging from 10 mg/kg to 20 mg/kg (Lovering, 1969;Cheema et al. 2020). Additionaly, the main origin of Pb pollution in sediments is the metal coating, leather whipping, burning coal and fossil fuels, waste pyromania and paint factories (Kushwaha et al., 2018;Ullah et al., 2017;Yousaf et al., 2017;Cheema et al., 2020 (Fig.8a). In the same way, the bulk Pb content stated as 1/Pb normalized values and the 206 Pb/ 207 Pb ratios (Lermi and Sunkari, 2020) showed that no Pb pollution of the samples and that present Pb originated from aerosol, gasoline and geogenic sources (Fig. 8b).

Multivariate statistical analyses
Factor analysis (FA) and Pearson's correlation matrix (PCM) using principial component analysis (PCA) were applied in this study. Pearson correlation analysis results were given in the

Source identification
It is known that heavy metal(loid)s originated from both anthropogenic and natural sources. In accordance with, the EF, Igeo, Pi and Pln values and multivariate statistical analyses, the results suggested two main sources of these metals/metalloid: (1) all the heavy metal(loid)s (except As) in the Trabzon area, Zn, Pb, Cu, Co and Mo in the Giresun area, Zn, Pb, Cu and Mo in the Gümüşhane area mainly originated from natural sources, as their EF, Igeo, Pi and Pln values were lower than threshold values; (2) According to the EF, Igeo, Pi and Pln values; the sandstones samples in the Trabzon area were partially polluted by As, in the Giresun area were significantly polluted by Ni, partially polluted by As and V, and in the Gümüşhane area were significantly polluted by Ni and As, partially polluted by V and Co. However, the concentrations of the Ni were significantly correlated with that of the Al, Zn, Cu in the Giresun, and Co, Mo, V in the Gümüşhane area. On account of this, Ni, V and Co were probably originated from natural sources. With respect to Sc/Th vs Co/Th diagram, which is commonly used to separate between the felsic and the mafic sources of the sandstones (Das et al., 2008;Singh, 2010;Peng et al., 2011;Fang et al., 2021), the sandstones samples were mainly plotted in the mafic field (Fig. 10). In this context, Co, V and Ni enrichment may have occurred due to mafic source rocks.
(3) The concentrations of As were a positive correlated with Pb and Mo in the Giresun area, and with Pb in the Gümüşhane area, likely indicating that anthropogenic sources, such as coal and gasoline combustion.
When 206 Pb/ 207 Pb ratio is plotted relative to 208 Pb/ 206 Pb ratio, it can show general trend that can be used to describe Pb pollution sources (Monna et al., 2000;Potra et al., 2017). In such diagrams, if the data constitutes several groups, multiple Pb sources can be extract and linear inclinations mean mixed Pb sources. However, if the data constitutes a group, only one source of Pb can be invited (Hurst et al., 1996). According to 206 Pb/ 207 Pb vs 208 Pb/ 206 Pb diagram, majority of the samples were located in the natural source field, and one sample each from the Trabzon Giresun and Gümüşhane areas located in the fields of U.S. gasoline (Teutsch et al., 2001) and Central Europe coal (Novak et al. 2003), and one sample each from the Trabzon and Giresun areas located away from the identified Pb sources (Fig. 8a) and these probably originated from other rare sources. The diagram of the 1/Pb normalized values and 206 Pb/ 207 Pb ratios also confirmed the above (Fig. 8b). In this diagram, all the samples were clustered in geogenic + aerosol + gasoline originated lead field. Additionally, the concentrations of Pb were a positive correlated with As in the Gümüşhane and Giresun areas, indicating that the source of As could be coal burning and gasoline combustion. The study area (Dağbaşı) in Trabzon region is sparsely populated and intensive farming activity. On account of this, besides burning coal unconscious use of fertilizers, insecticide and weed killers during farming activities may have also caused partially As enrichment in this sandstones. The studied areas (Çamlıyayla and Evliyatepesi) in Giresun and (Musalla, Pirahmet and Mescitli) in Gümüşhane were where the human population more intense than Dağbaşı (Trabzon), so the source of As could be burning coal and gasoline combustion. The enrichment of As in these areas was primary about atmospheric deposition of burning coal and gasoline combustion. The As in the atmosphere, likely amalgamated studied sediments with rainwater, and this element was filtered through porous sandstones and gathered in places.

Paleoclimate conditions
The concentrations of some trace and major elements in the sedimentary rocks indicate the paleoclimates changes (Hu et al., 2017). For instance, Ni, Mn, Fe, Cr and V are relatively enriched under moist climate conditions, whereas Mg, Sr, K, Ba, Na and Ca are enriched under arid climate conditions (Cao et al., 2012). C-value as proxy for climatic prosses has been applied in previous studies (C-value = Σ(Fe+Mn+Cr+Ni+V+Co)/ Σ(Ca+Mg+Na+K+Sr+Ba)) (Zhao et al., 2007;Fu et al., 2016;Wang et al., 2017;Vd'ačný et al., 2019;Bal Akkoca et al., 2019;). The C-values for the Trabzon area range from 0.03 and 0.11 (on mean 0.06), for the Giresun area range from 0.34 and 0.61 (on mean 0.50), for Gümüşhane area range from 0.09 and 1.24 (on mean 0.46) ( Table 1). The C-values indicated arid paleoclimate conditions for the Trabzon area, semiarid to semimoist paleoclimate conditions for the Giresun area and arid to moist paleoclimate conditions for the Gümüşhane area (Fig. 11). The highest concentrations of heavy metal(loid)s were measured in the Gümüşhane area, followed by Giresun and Trabzon areas (Fig. 11). It is meant that the heavy metal(loid)s deposited more in humid climatic conditions than arid climatic conditions.

Paleo-redox conditions
The content and ratio of some redox-sensitive trace elements (e.g. Ni, Th, Mo, V, Co, U and V/(V+Ni), Ni/Co, Th/U) can determine redox conditions of clastic rocks (Yarincik et al., 2000;Lewan and Maynard, 1982;Jones and Manning, 1994;Algae and Maynard, 2004). In sediments, U is depleted under oxidizing conditions and enriched under reducing conditions, while Th is not influenced by redox conditions (Kimura and Watanabe, 2001).
U/Th ratio is lower than 0.75, pointing out an oxidation environment, U/Th ratios are between 0.75 and 1.25, reflecting a weak oxidation environment, and a ratio > 1.25 points to a reducing environment (Jones and Manning, 1994) (Table 8) (Table 8). It is known that V is a redox-sensitive chemical element with a trend to be lesser concentrated in sediments principial oxic water (Lewan and Maynard, 1982). Thus, Mo content, Ni/Co and V/(V+Ni) ratios were used as paleo-redox proxies for defining the redox conditions (Vd'ačný et al., 2019). Wang et al. (2017) suggested that Ni/Co ratios < 5 reflect an oxidation environment, range from 5 to 7, pointing out a weak oxidation environment, and > 7 reflect a reducing environment. V/(V+Ni) ratio <0.6 exhibits an oxidation environment, 0.64 -0.84 indicates a weak oxidation environment, and > 0.84 implies reducing environment   (Table 8) (Fig. 12a, b) confirmed oxic and weak oxic conditions during deposition for the analyzed samples.

Paleo-salinity
Paleo-salinity is a significant cursor of the depositional environment because the salinity of ancient water is determined by paleo-salinity degree. The ratio and concept of some elements such as Sr/Ba and Log m (Log(100x(MgO/Al2O3)) (Jones and Manning, 1994) can define the marine and continental facies of depositional environment and paleo-salinity (Li et al., 2020). Log m value and Sr/Ba ratio decrease from marine to continental facies. The Log m value <0 indicates nonmarine facies, while 0-1 represents transition facies, and >1 shows marine facies (Jones and Manning, 1994). Wang et al. (2017) suggested that the Sr/Ba < 0.6 represents nonmarine facies, 0.6 -1.0 indicates transition facies, and >1.0 represents marine facies (

Conclusions
In this paper, the concentrations of heavy metal(loid)s (Zn, Pb, Cu, Ni, As, Co, Mo and V), mean grain size (Mz) and TOC were analyzed in the Late Cretaceous aged sandstones collected from Trabzon, Giresun and Gümüşhane areas (Eastern Pontides).
The mean concentrations of the heavy metal(loid) followed in the descending order of V>Zn>Ni>Pb>Cu>Co>As>Mo in the Trabzon area, V>Zn>Ni>Cu>Co>Pb>As>Mo in the Giresun area, V>Zn>Ni> Cu>Co>Pb>As>Mo in the Gümüşhane area. According to mean EF values, all heavy metal(loid)s were not enriched in the sandstone samples for Trabzon area, Ni was enriched in the samples for Giresun area and As and Ni for Gümüşhane area. According to Igeo values, the sandstones was unpollutedmoderately polluted by As in the studied three sites. Pln values indicated that investigated sandstones were low polluted by As and Cu for Trabzon area, low polluted by Cu and Co, moderately polluted by As, and strongly polluted by Ni for Giresun area, and low polluted by Zn, Pb and Cu, moderately polluted by Co and V, strongly polluted by Ni and As for Gümüşhane area.
The results of multivariate statistical analyses in the samples indicated that the accumulation of Zn, Cu, Ni, Co, V, Mo and mostly Pb was derived from geogenic sources, whereas As and partly Pb (for Gümüşhane and Giresun) might be derived from anthropogenic sources (atmospheric deposit for Giresun and Gümüşhane, agriculture for Trabzon). Pb isotopic tracing supported multivariate analysis results. Lead isotopic analysis indicated that two signatures related to the sources of heavy metal(loid); first signature indicated that natural Pb was the main sources and second signature showed that anthropogenic Pb was the minor sources including atmospheric deposits (coal burning, gasoline combustion and aerosol) and may be agriculture activities.
Paleoclimate index showed arid climatic conditions for Trabzon area, semiarid to semimoist climatic conditions for Giresun area and arid to moist climatic conditions for Gümüşhane area during deposition of the studied sandstones. These sandstones were deposited oxicweak oxic water column and in the transitionmarine environment. According to the data obtained in this study, heavy metal(loid)s accumulated more in moist climatic conditions than arid climatic conditions.      Boundaries for different redox environments are from Jones and Manning (1994) and Xu et al. (2012).